1. Field of the Invention
This invention relates to a sheet feeder used with an image formation apparatus such as a copier or a printer and in particularly to improvement in a sheet feeder of the type wherein sheets are pushed up and supported on a moving bottom plate and are delivered in order from the top, and an image formation apparatus using the sheet feeder.
2. Description of the Related Art
Generally, in an image formation apparatus such as a copier or a printer, an image formed in an image formation section is transferred to a sheet of paper, etc., and thus usually a sheet feeder for supplying sheets to the image formation section is provided.
Available as this kind of sheet feeder in a related art is a sheet feeder having a sheet tray for storing sheets and a sheet delivery unit disposed above the sheet tray for delivering the sheets in order from the top through the sheet delivery unit.
The sheet delivery unit includes, for example, a pickup roll for delivering sheets and a handling mechanism (for example, made up of a feed roll and a retard roll) for handing the delivered sheets one at a time.
By the way, this kind of sheet feeder adopts a bottom plate lift system to deliver the sheets in the sheet tray.
In the bottom plate lift system, a bottom plate is liftably disposed on the bottom of a sheet tray and sheets are pushed up and supported on the bottom plate, whereby the top sheet is pressed against a pickup roll of a sheet delivery unit, and the sheet is delivered by means of the pickup roll and then is introduced into a handling mechanism for handling the sheets one at a time.
By the way, in this kind of bottom plate lift system, an apparatus, for example, including a drive source such as a motor, and a drive force transmission mechanism for transmitting the drive source from the drive source to a bottom plate, for lifting up the bottom plate in response to the remaining amount of sheets stacked on the bottom plate, is already proposed (for example, refer to JP-A-5-4733, JP-A-5-229674, etc.,). Since this kind of bottom plate lift system requires the drive source and its drive force transmission mechanism and further a sheet remaining amount sensing system, the costs are increased accordingly.
An apparatus wherein the drive source of a motor, etc., is replaced with an elastic member of a spring, etc., is also already proposed (for example, refer to JP-A-11-29226); however, to handle sheets of various sizes and paper qualities, the lift amount of the bottom plate varies as the sheet weight differs, and the sheet delivery operation of a pickup roll easily becomes unstable.
Then, to solve such a problem effectively, the bottom plate lift amount must be controlled delicately, leading to a technical problem of complicating the control mechanism of the bottom plate lift amount.
It is therefore an object of the invention to provide a sheet feeder for making it possible to stabilize the sheet delivery operation in a simple configuration and an image formation apparatus using the sheet feeder.
According to the invention, there is provided a sheet feeder including: a sheet tray for storing sheets; a moving bottom plate being disposed on a bottom of the sheet tray for stacking the sheets; an elastic member for elastically urging the moving bottom plate; a sheet delivery unit having a sheet delivery member being placed in contact with the sheet stored in the sheet tray for delivering the sheets in order from the top through the sheet delivery member; and an interlock mechanism for regulating a move of the moving bottom plate in response to the sheet stack amount so as to keep substantially constant the contact relationship between the top sheet position of the sheets stored in the sheet tray and the sheet delivery member.